1. Field of the Invention
This invention relates generally to an apparatus, process, and the product produced therefrom for constructing a spunbond, non-woven web from thermoplastic polymers producing filaments of reduced diameter and improved uniformity at an increased production rate, and specifically, to an apparatus and process for heating and extruding thermoplastic materials through a spinneret, forming filaments of desired deniers by strategically positioning the drawing unit below the spinneret at a critical distance to increase filament velocity by reducing filament air drag and increasing form drag on the filaments by selective drawing unit slot length and air turbulence below the drawing unit slot to produce a filament of a desired diameter and molecular orientation and at an improved production rate, and the resultant spunbond product. A water spray for cooling may also be employed.
2. Description of the Prior Art
Devices for producing non-woven thermoplastic fabric webs from extruded polymers through a spinneret that form a vertically oriented curtain with downward advancing filaments and air quenching the filaments in conjunction with a suction-type drawing or attenuating air slot are well known in the art. U.S. Pat. No. 5,292,239 discloses a device that reduces significant turbulence in the airflow to uniformly and consistently apply the drawing force to the filaments, which results in a uniform and predictable draw of the filaments. U.S. Pat. No. 3,802,817 discloses a sucker apparatus positioned in a selected distance below the spinneret using jet streams having velocity in the range of turbulent flow to produce fine non-woven fleeces. U.S. Pat. No. 4,064,605 and European Patent Application No. 0230541 disclose examples of the formation of non-woven fabrics.
Conventionally, thermoplastic polymers such as polypropylene, polyethylene, polyester, nylon, and blends thereof are utilized. In the first step, the polymer is melted and extruded through a spinneret to form the vertically oriented curtain of downwardly advancing filaments. The filaments are then passed through the quench chamber where they are cooled down by chilled air, reaching a temperature at which the crystallization of the filament starts, resulting in the solidification of the filaments. A drawing unit located in a fixed position below the quench chamber acts as a suction having an air slot where compressed air is introduced into the slot creating a downward force on the filaments in the slot, and a rapidly moving downstream of air in the slot. This air stream creates a drawing force on the filaments, causing them to be attenuated or stretched above the drawing unit and exit the bottom of the slot where the filaments are deposited on a moving conveyor belt to form a continuous web of the filaments. The filaments of the web are then joined to each other through conventional techniques.
Providing for conventional construction of the filaments, typically filaments of 1.5 to 6 deniers or higher were produced. Using conventional methods, the hot filaments leaving the spinneret typically were immediately cooled to ambient temperature and solidified and then subjected to the drawing unit. According to a prior proposal, when the length of the filament traveling through the air is shorter than a specified value selected based on the throughput (gram per hole per minute) used, the extruded filaments will contact with a solid constituent of the drawing unit in advance of solidification of the filaments, resulting in development of filament breakage or damage. In other words, even though the prior art produces suitable non-woven webs, their production is limited by the ability to cool down and solidify the filaments in a predetermined length at appropriate throughput. The filament spinning speed reached in the prior art is in the range of 3,000 to 3,500 meters per minute.
Although the conventional method and apparatus produce suitable non-woven webs, the final product could be greatly improved and better fabric can be produced for any given polymer consisting of lower denier filaments. A thinner filament for any given throughput produces more surface area and more length per unit weight. A polypropylene spunbonded fabric with filaments of 0.1 to 5.0 or higher deniers would be desirable.
It is also desirable that a uniformity of filament denier and tensile properties be consistent so that the resulting fabric web has a uniform quality. Applicant has also determined that stress-induced crystallization in the spinning process (before solidification by cool-down) results in a stronger, higher tensile strength filament. The present invention provides for a higher rate of stress-induced crystallization.
Examples of end uses for the fabric web could be but are not limited to filtration materials, diaper covers and medical and personal hygiene products requiring liquid vapor barriers that are breathable and have air permeability.
With the present invention, a process for producing a superior quality non-woven web at much higher production and lower cost can be achieved. The core of the invention is to create an optimum situation wherein a much higher filament velocity (compared with that of conventional technologies) is achieved by selecting a minimum distance from the spinneret to the drawing unit based on the operating variables determined by such basic factors as the materials processed, the filament denier required, the throughput used, therefore resulting in a reduced air viscous friction drag associated with the length of filaments traveling with high velocity between the spinneret and the drawing unit and hence a reduced spin line tension, coupled with a higher drawing force created by the drawing unit with an optimal short slot vertical length to maximize the combination of the forces in terms of both the friction drag between air stream and filaments within the slot and the form drag underneath the slot.
The fiber velocity in the spunbond process is ultimately determined by the spin line force balance which, in the case of high speed spinning, can be reduced to the equation as follows: EQU F.sub.ext =F.sub.inert +F.sub.drag
The force F.sub.ext is the external inert force pulling down the filaments. The force F.sub.inert is the inertia force which opposes the acceleration of the filaments and the force F.sub.drag is the air resistance produced by skin friction of the filaments traveling with high velocity in the air. Based on the spin line force balance, there are two ways to increase the filament spinning speed, that is, by increasing the force F.sub.ext and/or decreasing the force F.sub.drag. The force F.sub.drag is linearly proportional to the length of the spin line, therefore shortening the distance between the spinneret and the drawing unit will reduce the air resistance accordingly.
As to the F.sub.ext, unlike the fiber production where the F.sub.ext is supplied by a mechanical take-up reel or bobbin, the downward pulling force in the spunbond process on the filaments is created by the drawing unit which employs one or more streams of high velocity air directed downwardly inside the drawing unit slot in the direction of filament travel and interaction between the air stream generated inside the drawing unit and in accordance with the invention, the air flow below the drawing unit and the filaments. There are two types of such interaction: The first type arises from the viscous friction resulting from the differences in velocity between the filaments within the drawing unit and the drawing unit air stream in that the air stream with higher velocity pulls the filaments of slower speed downwardly. Therefore, the filament speed will always be lower than that of the air stream. The pull frictional force by viscous friction has almost a linear relationship with the generated air stream nozzle air velocity. The second type is the so-called "form" drag caused by the filaments "flapping" or "waving" in the airflow field below the drawing unit. It is very clear from the discussion above that the effectiveness of the draw unit (air gun or slot) in terms of affecting the filament velocity reached in the spunbond process depends strongly on the way the drawing unit produces the drag force.
The short draw slot used in the present invention is the most effective one producing the maximum drag force by utilizing an optimum combination of creating viscous friction drag within the drawing unit slot and the "form" drag underneath the slot.
The increased net drawing force not only produces thinner filaments at higher filament spinning speed, but also creates a stronger stress-induced crystallization effect, causing the on-line crystallization of filaments to occur earlier along the spinline at higher temperature and rate. Correspondingly, the filaments are solidified earlier at higher temperature, thereby resulting in less quench capacity needed or a higher mass throughput that can be used with the same quench capacity. Up to 90 to 95 percent reduction of the air drag associated with the length of filaments between the drawing unit and the spinneret can be achieved by moving the drawing unit from a conventional distance of 3 to 5 meters from the spinneret to 0.05 to 1.5 meters, giving rise to the possibility of producing finer filaments at a higher production rate. For the coarse denier filaments (5 to 20), the distance should be larger (up to 1.5 meters) than for fine deniers (0.1-5) best achieved in a range of 0.2 to 0.9 meters. By determining the most efficient filament distance between the spinneret and the drawing unit, the diameter of the filaments can be controlled in such a way that sticking among filaments in contact can be avoided, the temperature of the filaments remains as high as possible before the filaments enter the drawing unit, reducing the viscosity of the filaments being drawn and consequently facilitating the attenuation of the filament, resulting in filaments having much smaller diameters. The distance between the web forming table and the drawing unit can also be adjusted in order to form a non-woven web which has desired uniformity with other mechanical properties.
A quench chamber is normally used to initially cool the filaments. A water mist or atomized water spray may be added for cooling and interacting in the process to improve the filament uniformity and production. The water mist improves the process, but the basic apparatus and process will work without the water mist solely by the reduced separation of the spinneret and the drawing unit, with cooling air as a quench.
In terms of filament spinning speed, 4500 meters per minute for polyethylene terephthalate (PET) and 3500 meters per minute for polypropylene (PP) are achievable in the prior art and in commercial production today. With the Applicant's invention, Applicant believes that 6000-8000 or more meters per minute for PET and 4500-6500 or more meters per minute for PP have been achieved. Applicant has been able to produce filaments (5 to 10 microns at spunbond production rates 70 to 150 Kg/H/M width), which is far beyond the capability of conventional production technology.
Although it is advantageous to provide the machine with the adjustability of the distance between the spinneret and the draw unit so that the maximum filament velocity for a given material such as polypropylene or polyester and a given throughput and filament denier can be readily obtained with adjusting of the draw unit to an optimum distance to the spinneret, it is highly possible that for a fixed product in terms of material processed and filament denier required, an optimum machine setup and processing conditions can be predetermined to produce the desired product with maximum filament velocity, resulting in the fact that the machine's adjustability of the distance between the spinneret and the draw unit will no longer be necessary.
A stable process can be obtained wherein 4.5 denier PET filaments are produced at 4.0 ghm with the drawing unit positioned at an optimum distance from the spinneret using 75 psig of air pressure. Applicant has found that Applicant can use fixed distances between 5 and 150 cm and optimally between 20 to 90 cm separation for fine deniers (0.1-5) and up to 150 cm for coarse deniers (5-20) between the spinneret and the drawing unit.
There are two distinct changes occurring for the on-line diameter profile as the filament spinning velocity increases. First, the rate of reduction in diameter of the melt thread in the upper region of the spinline increases. In other words, the melt thread is thinning much faster at a higher spinning speed, creating more surface area to be cooled. Secondly, the invention improves the position where the filament starts to solidify due to the stress-induced crystallization effect, which moves up the solidification point towards the spinneret. The higher the filament speed, the less the cooling is needed (shorter quench chamber). The drawing unit can be permanently positioned closer along the spinline without causing interruption of the process because the filaments are well solidified before entering the slot of the drawing unit where contacts among filaments are made. Because the distance between the spinneret and the drawing unit is decreased, the drag force F.sub.drag, which is associated with the length of filaments (dZ) traveling at high speed between spinneret and drawing unit will proportionally be reduced, resulting in increasing inertial force F.sub.inert, which leads to even higher filament speed, further thinner filaments and higher solidifying temperature. This in turn allows the drawing unit to be positioned at an optimum minimal distance to the spinneret for minimum viscous air drag for a given filament material and desired denier. Applicant's results show that depending on the material to be processed and the throughput (gram per hole per minute, referred to as ghm from now on) to be used, the drawing unit can be as close as 5 to 90 cm for fine deniers (0.1 to 5.0) and up to 150 cm for coarse deniers (5-20) to the spinneret at a throughput of up to 4 ghm or more, compared with 3 to 5 meters being used in commercial production today. Thus up to 90 to 95 percent of reduction in spinline length is achieved with significant improvement on-the output of the process in terms of fineness of filaments that can be produced at a given production rate. The closer the drawing unit to the spinneret, the higher the temperature at which filaments are being drawn hence the lower the elongational viscosity will be, which is inversely proportional to the elongation rate. That is, with lower elongational viscosity, higher elongation rate (higher filament speed) can be achieved under the same drawing force.